This invention relates to a method of detecting back corona in electrostatic precipitators, measuring parameters, which indicate back corona susceptibility, precipitation performance and electrode contamination, and determine the back corona current and conductivity in order to control the precipitator and associated plant to limit back corona.
An electrostatic precipitator is a device which uses electricity to collect dust particles suspended in a gas. The device consists of two sets of electrodes, one of which is energised from a high voltage electricity supply while the second is earthed. The gas-particle mixture is passed between the two electrodes. The particles are charged by ions created by a corona about the energised, emitter electrode. The particles are then attracted to the collector electrode by the electric field.
Each precipitator may have one or more electrical zones, each energized from a single high voltage supply. Each electrical zone normally has many emitter electrodes connected in parallel and many collector electrodes connected to earth by the precipitator frame. This may result in an extremely large and expensive device.
The emitter electrodes are energised using a power control unit and a transformer to provide the high voltage necessary. FIG. 1 depicts a schematic diagram of a typical electrostatic precipitator energisation system. The power control unit regulates the primary A.C. input to the transformer using a silicon controlled rectifier phase angle controller or a magnetic amplifier. The high voltage transfromer input is adjusted by varying the control unit output using a reference or setpoint signal. Adjustment of the control unit reference signal will cause both the emitter voltage and emitter current to change. The emitter voltage level signal and emitter current level signal are available, or can be obtained using voltage divider resistor networks, for each electrical section of the precipitator.
As the control unit reference signal is increased from zero, the emitter voltage increases but the emitter current remains at zero. At a certain emitter voltage, termed the "Emitter Corona Onset Voltage", the emitter current commences. Further increases in the control unit reference signal will cause the emitter current to increase. The emitter voltage may increase or decrease depending on the precipitator conditions and energisation level. FIGS. 2A-2C depict the emitter voltage waveform and emitter current waveform for low, medium and high energisation, or control unit reference signal, levels on a typical precipitator with 50 Hz. A.C. energisation. The emitter current is a pulsed waveform, coincident with increasing emitter voltage, while the emitter voltage has an A.C. component superimposed on a D.C. level.
Back corona is the term used to describe the gaseous breakdown which occurs in the collected dust layer. The breakdown is a result of intense electric fields created in the collected dust by the conduction of charge through the highly resistive dust. The collection efficiency of the electrostatic precipitator is reduced by the presence of back corona. The detection and limitation of back corona is important when highly resistive dusts, such as Queensland coal fly ash, are being collected in an electrostatic precipitator.
As the energisation level of the electrostatic precipitator is increased, the precipitation of particles improves due to the higher inter-electrode electric fields and particle charge. Once sufficient charge flow exists for back corona to form, the detrimental effects caused by back corona will restrict the improvement attained from increasing energisation. The back corona effects, increasing rapidly with energisation, will cause a reduction in the electrostatic precipitator's collection efficiency. A maximum efficiency will occur at or just above the back corona formation energisation level.
In order to prevent back corona, gases are introduced into the precipitator intake gas before it reaches the precipitator. The use of substances, such as Sulphurtrioxide, Ammonia or Steam, to improve precipitator performance by reducing or eliminating back corona has been practiced for some time. Since the cost of some of the substances used is high, the operating cost of the precipitator can increase dramatically if the addition of the conditioning agent is not regulated properly.